Process for melt spinning hollow filaments



July 5, 1966 P. T. SCOTT 3,259,674

PROCESS FOR MELT SPINNING HOLLOW FILAMENTS Filed May 5, 1965 WWW INVENTOR PAU L T. SCOTT BY ZMWV ATTORNEY United States Patent 3,259,674 PROCESS FOR MELT SPINNING HOLLOW FILAMENTS Paul T. Scott, Kinston, N.C., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed May 5, 1965, Ser. No. 453,463 3 Claims. (Cl. 26453) This invention relates to extrusion-spinning of lowdensity synthetic textile fibers and, more particularly, to a method for producing melt-spun, hollow, textile fibers of either the tube-like or bubble-filled types.

The prior art teaches many methods of generating voids in synthetic fibers to produce low-density products. Typical recesses for producing bubble-containing fibers involve either the use of solid or liquid blowing agents which are mixed with the polymer prior to spinning, and which form a gas upon filament extrusion, or the use of gas which is pumped into the polymer under high pressure before extrusion and which expands upon filament extrusion. The former method has the disadvantages of necessitating an expenditure for blowing agents and the fact that the fiber is contaminated by the blowing agents or decomposition products thereof, rendering the fiber unsuitable for purposes which require a high-purity product. The latter method entails the use of high-pressure equipment which is expensive and is difficult to maintain in good operating condition. The prior art also teaches methods for producing hollow, tube-like fibers which are spun by means of specially constructed spinnerets. In order to prevent the tube from collapsing, these methods also include the use of either blowing agents, which carry their particular shortcomings noted above, or highpressure gas supplied to the center of an annular orifice. A longitudinally uninterrupted center void in the fiber is produced by these methods.

The present invention provides a process for easily and economically producing tube-like or bubble-filled hollow textile fibers, the hollow content of which can be readily regulated. A further provision is a method for producing low-density, tube-like or bubble-filled textile fibers which does not necessitate the use of any additional chemical constituents, and therefore avoids undesirable byproducts and contaminants. The invention also provides a method for producing low-density textile fibers which does not necessitate expensive high-pressure or additive equipment. Other advantages of the invention will become apparent hereinafter.

It has now been found that hollow fibers can be formed by pyrolyzing a portion of the molten polymer as it is fed to the spinneret through the polymer flow channels upstream of the spinneret. This can be accomplished with closely associated low-resistance electrodes connected at their inner ends by inert high resistance wire. The wire is heated by electrical current passing through it. A small portion of the polymer coming in contact with or passing near to this wire is pyrolyzed to form gas either at that point or as the polymer is extruded through the spinneret orifice. If hollow tube-like yarn is desired, a center-obstructed-orifice spinneret, as disclosed in Scott US. Patent No. 3,095,258, dated June 25, 1963, is used. The preparation of such spinnerets is disclosed in 'Burke, Jr. et al. US. Patent No. 3,141,358, dated July 21, 1964. If a bubble-containing yarn is desired a conventional spinneret is used, but solid particles must be included in the polymer composition to serve as bubble-forming nuclei.

The drawing is a schematic illustration of melt-spinning apparatus suitable for practicing the invention, shown partially in vertical cross-section taken along the central axis,

3,259,674 Patented July 5, 1966 ice As shown in the drawing, molten polymer is supplied by conventional means, such as a screw melter, through conduit 1 to metering pump 2, flows through a conventional filter pack 3, and is extruded through spinneret 4 to form filaments 5. The pyrolyzing device 6 is installed in conduit 1. It maybe an automobile spark plug having high resistance wire 7 silver-soldered across the terminals. The external terminals of this device are connected to a low voltage current source (not shown) through regulator 8. The wire 7 is heated by the electric current to pyrolyze the polymer with formation of gas. The gas forms bubbles 9 in the flowing polymer when the molten polymer contains suitable solid particles to act as bubble-form-ing sites. Otherwise, when a spinneret having center-obstructed orifices is used, the gas forms an uninterrupted central void as the polymer flows through the spinneret.

It is an important feature of the present invention that the size of the hollow portion of the fiber, which results from the pyrolysis in the polymer, is adjusted simply by regulating the electric voltage applied to the said wire. Thus, if a larger hollow content is desired, a greater amount of voltage is applied and the resulting increased heat which is generated decomposes a greater portion of the polymer to form the gas-filled voids.

If bubble-containing yarn is to be produced in accordance with this invention it is imperative that a certain amount and size of solid particles be present in the polymer which is spun. This is because the solid particles are needed to act as nucleating agents or sites of bubble formation. Preferably the conventionally used pigment and a special nucleating agent are used in combination, the proportion of pigment to other nucleating agent in the polymer being determined by the degree of delustering or coloring desired in the product. Pigments such as titanium dioxide (TiO may also act as the nucleating agents in the practice of this invention, although the TiO or other pigment is preferably combined with sodium acetate (NaOAc) or potassium acetate (KOAc) nucleating agents. The important rule is that enough solid particles of sufficiently small size be added so that the many small bubbles will nucleate, and yet so that the concentration of the small particles is not so great as to hamper spinning continuity or give poor quality delustered yarn.

For producing bubble-containing yarn, approximately 0.2 to 1.0 weight percent pigment, or combination of pigment and other solid nucleating agent, is used. The pigment, or pigment combination, is approximately 0.3 to 0.6 micron particle size. Preferably, 0.6 to 0.9 percent by weight pigrnent of particle size 0.4 to 0.5 micron is used.

In the production of hollow tube-like yarn in accordance with the present invention a spinneret is used which contains an obstruction in the center of the orifice, such as one of the type referenced above. In this case no pigment or solid nucleating agent is needed because the orifice obstruction acts as a nucleating agent to produce a longitudinally uninterrupted center void in the fibers.

Accordingly, in the practice of this invention, a small portion of the polymer is pyrolyzed at a short distance upstream of the spinneret, preferably by means of a hot wire which is in contact with the polymer, but pyrolysis can also be effected by heat applied externally to a portion of the polymer piping system. The molten polymer, containing a small amount of gas generated by the pyrolysis, is then extruded through the spinneret and quenched to solidify the filaments. A bubble-containing or hollow tubular fiber is produced; the former being accomplished by the use of a conventional spinneret and the inclusion of solid nucleating particles in the polymer, and the latter being accomplished by the use of a special spinneret (described by reference hereinabove) without nucleating particles in the polymer.

The hot wire which effects the polymer pyrolysis is preferably located approximately 0.5 to 2 meters upstream of the spinneret. It is preferably 0.25 to 1.00 mm. in diameter and may be connected to the terminals of an automobile spark plug which are spaced approximately 3 to 10 mm., apart. This wire is heated by applying suitable voltage to the spark plug terminals. The preferred voltage in this instance is in the range of 0.5 to 2.0 volts.

The invention is further illustrated by the following examples of preferred embodiments, although it will be understood that these examples are included merely for the purpose of illustration, and are not intended to limit the scope of the invention.

The relative viscosity of the yarn (RV) as used therein is the ratio of the viscosity of a 10% solution of polyethylene terephthalate in a mixture of 10 parts of phenol and 7 parts of 2,4,6-trichlorophenl (by weight) to the viscosity of the phenol trichlorophenol mixture, per se, measured in the same units at 25 C.

Example I Polyethylene terephthalate, having a relative viscosity of 3512, containing no added pigments, is extruded through a center-obstructed-orifice spinneret at a rate of 3.85 pounds/hr. (29.3 grn./min.) at a temperature of 295 C. A 0.8 mm. thick Nichrome wire, 22 mm. in length which had been silver-soldered across the electrodes of an automobile spark plug, spaced 9 mm. apart, is installed in the polymer melt, 1.5 meters upstream of the spinneret. Electrical current is applied to the spark plug so that the voltage is varied from 0 to 1.7 volts.

The following table relates the effects of the varying voltage to the hollow content of the tubular fiber produced in this example. It is apparent that when no voltage is applied no void is created, and the hollow content of the fiber is directly related to the voltage applied and the consequent heat of the wire.

TABLE I Estimated I111. Percent Voltage gas/hr. SIP 1 Hollow in Fiber 1 Gas volume at standard temperature and pressure conditions.

Example II Melts of polyethylene terephthalate, having a relative viscosity of 24.5 to 25.2, containing 1.0% by weight KOAc of an average particle size of 0.1-0.5 micron is .spun using a conventional spinneret of orifice size 0.229 mm. (9 mil) in diameter, at a rate of 3.6 lb./hr. (27.2 gm./mm.) and a temperature of 298i5 C. to produce 34-filament yarn of deniers ranging from 200 to 210. A 25-mrn. platinum wire which had been silver-soldered across the terminals (9 opening) of an automobile TABLE 2 Subjective Rating of Yarn Polymer,

Yarn, RV

Tenacity (g-l denier Elongation, Percent Denier No bubbles. Few bubbles" IONMM weep-mow:

In addition to polyethylene terephthalate, other polyesters, polyamides, copolymers of polyesters and copolymers of polyamides and, in general, any synthetic, meltspun, highly polymeric textile fiber may be produced in accordance with this invention.

All types of pigments which are used in normal textile fiber production may be used in the practice of this invention. Thus, TiO Zns, BaSO and SiO may all be used.

The products of this invention are lightweight, hollow, tube-like or bubble-containing, fibers. Tube-like fibers have a useful application in gas separation operations, as for example, in the separation of helium from natural gas or oxygen from air. Hollow, tube-like fibers are also used for producing woven or knitted fabrics with increased bulk and covering power. The bubble-containing yarns produced in accord with this invention have useful application in fabrics. These fabrics are lightweight and have good covering power. An economic advantage exists in the utilization of these fibers for fabrics in that less raw materials are used for given fabric construction.

The process of this invention possesses advantages over the processes of the prior art, in that no foreign materials such as blowing agents need be added, and thus no foreign residues remain in the finished product. Another distinct advantage lies in the fact that the hollow content can be readily adjusted without the aid of expensive high-pressure equipment or additive systems.

Since many different embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited by the specific illustrations except to the extent defined in the followingclaims.

I claim:

1. In the production of melt-spun textile fibers of synthetic linear polymer by extruding a melt of the polymer through a spinneret to form filaments, the process which comprises forming bubbles in the polymer by pyrolyzing a portion of the molten polymer flowing to the spinneret to form gas, then extruding the bubble-containing polymer and solidifying the extruded polymer to form hollow filaments.

2. A process as defined in claim 1 wherein the bubbles are formed in the polymer by flowing the molten polymer past a high resistance wire upstream of the spinneret and heating the wire by passing electrical current through the wire to pyrolyze a portion of the molten polymer into gas.

3. A process as defined in claim 1 wherein the bubbles are formed in the polymer by flowing the molten polymer past a high resistance wire upstream of the spinneret,

filaments.

UNITED References Cited by the Examiner STATES PATENTS Snelling 264-209 Kosuge 264-209 Lindemann 264-167 Wiczer 264167 Bottomley 264-167 6 References Cited by the Applicant UNITED STATES PATENTS 1,487,807 3/ 1924 Rousset. 5 3,095,258 6/1963 Scott.

3,141,358 7/1964 Burk et a1.

FOREIGN PATENTS 467,241 10/1928 Germany.

10 ALFRED L. LEAVITT, Primary Examiner.

ROBERT F. WHITE, Examiner.

R. B. MOFFITT, Assistant Examiner. 

1. IN THE PRODUCTION OF MELT-SPUN TEXTILE FIBERS OF SYNTHETIC LINEAR POLYMER BY EXTENDING A MELT OF THE POLYMER THROUGH A SPINNERET TO FORM FILAMENTS, THE PROCESS WHICH COMPRISES FORMING BUBBLES IN THE POLYMER BY PYROLYZING A PORTION OF THE MOLTEN POLYMER FLOWING TO THE SPINNERET TO FORM GAS, THEN EXTRUDING THE BUBBLE-CONTAINING POLYMER AND SOLIDIFYING THE EXTRUDED POLYMER TO FORM HOLLOW FILAOENTS. 